1. Field of the Invention
The present invention relates to an optical alignment apparatus adapted for use in the minifying-projection type of exposure system and the like.
2. Related Background Art
The above-mentioned type of optical apparatus has been known and used in the art. For example, such an optical alignment apparatus is disclosed in U.S. Pat. No. 4,402,596 specification. FIG. 1 of the accompanying drawings schematically shows the prior art apparatus in an exploded view of its optical path.
Referring to FIG. 1, the prior art optical alignment apparatus is used to accurately position an object to be exposed such as a wafer 1 relative to a reticle 4. The object 1 has as alignment mark A formed on its patterned surface 1a. The reticle 4 has an alignment reference mark B formed on its patterned surface 1 regarding to a projection objective lens 2. The off-axial rays passing through the alignment mark A and the alignment reference mark B form images on a screen 14 through first and second objective lenses 5 and 6, a collimator positive lens 7 and a focusing lens 13. Further, through first and second objective lenses 15 and 16 of a relay system, the rays form images of the marks A and B also on the image pickup surface 12a of an image-pickup unit 12.
The operator can establish a determined positional relation between the marks A and B by moving the object 1 or the reticle 4 while observing the images of the marks A and B through the image-pickup unit 12. In this manner, an accurate positioning the wafer 1 relative to the reticle 4, that is, an alignment between object and reticle can be achieved.
The prior art optical alignment apparatus as described above, however, some problems which will hereinafter be described.
In the prior art optical alignment apparatus, the first objective lens 5 must be positioned in such a position in which the lens 5 never blocks the exposures light emitted from a light source (not shown) during the time of exposure. The collimator positive lens 7 and the focusing lens 13 are so disposed that their optical axis is coaxial to the first and second objective lenses 5 and 6. The beam of rays passed through the marks A and B enters the marginal area of the first objective lens 5, and the principal rays of the beam passed through the marks A and B are obliquely incident on the screen 14 through the focusing lens 13. Under these conditions, it is not always assured to accurately detect the positions of the marks A and B. If there takes place any change in thickness of the reticle 4 which has the alignment reference mark B on its backside surface, the images of the marks A and B on the screen 14 will be not only blurred but also deviated. Furthermore, when the operator carries out a focusing operation in this state, the images of the marks A and B are shifted on the screen so that it may be no longer possible to accurately detect the positions of the marks A and B. This is the first problem involved in the prior art optical alignment apparatus.
The second problem of the prior art apparatus is found in the fact that it needs a large size of optical system as a whole. As previously mentioned, the beam of oblique rays transmitted through the objective lens 5 passes through the marginal area off the optical axis throughout all the optical elements following the objective lens 5. Therefore, these optical elements are required to have large diameters, which necessarily leads to a large optical system as a whole.